It is imperative to equip boilers and other circuits or systems containing a fluid under pressure with a safety valve designed to vent the pressure in a protected space to the atmosphere. In some cases a pressure relief valve in which the pressure within the space works only against a spring on the valve member, can be effective. In more sophisticated applications, however, such as in boilers for power plants, ships and industrial plants, or for the pressurized vessels of nuclear installations, it is desirable to provide a pilot-controlled valve in which the opening of the valve is ensured by the control of a pilot pressure which is applied to the valve member. A pilot valve can control the pilot pressure and the system may be operated in a master-slave relationship in which the valve provided at the pressure space is slaved to the pilot valve which can sense the pressure in this space.
The safety valve thus has the function of protecting a space under pressure and the equipment associated with that space when the pressure of the fluid in the space reaches a maximum predetermined pressure, thereby avoiding overpressures which may be a nuisance or dangerous should they arise in the protected space.
In general the safety valve is mounted on the space to be protected, e.g. on the boiler or other pressure vessel while the pilot system can be located away from the protected space, is connected to the protected space by a sensing conduit so as to be capable of responding to the pressure in the space, and is provided with means, namely the pilot valve, providing a fluid feed to the head of the safety valve to contribute a countervailing force to the forces of the fluid in the protected space. The safety valve comprises a valve body mounted on the pressure vessel and formed with a passage whose upper end constitutes a seat cooperating with a valve closure member. An orifice is also provided in the valve body to permit communication of the interior of the valve body to the atmosphere in an open position of the valve.
The valve member is mounted at one end of a rod sliding in the interior of the body and at the other end of which a command piston is provided in a cylinder fed with fluid from the pilot pressure source.
While such vales were found to be a major advance by comparison to the safety valves which had been used earlier, they have been found to have a number of drawbacks.
Firstly, the safety valve is of relatively complex construction because of the need for a command piston and cylinder. The sensitivity of such a cylinder made it necessary to provide thermal barriers to prevent the temperature within the vessel from adversely affecting the operation of the cylinder and the piston. Special attention had to be paid to the seals between the cylinder and the protected space or the atmosphere and which was susceptible to frictional wear and thus were compelled to offer a resistance to movement of the rod and the valve member and which were under considerable pressure and stress in the normally closed position of the valve when it was most necessary to avoid any significant contribution to resistance of displacement of the piston and the valve member.
The seals had to be designed to prevent hot fluid from entering the head of the valve and the construction of the valve was complicated further by the need to connect the valve to the atmosphere below the cylinder. As a consequence, the presence of the cylinder not only was a complicating factor in the design and operation of the valve, but also created an encumbrance which was undesirable.
Piloted safety valves were also provided heretofore with internal cylinders. These had the advantage of greater compactness than valves with external cylinders as previously described but had similar drawbacks when it was contemplated to use such valves at high temperatures.
French patent document No. 82 14 491 describes a safety valve whose support element for the valve member has two successive parts of different cross sections guided in two successive tubular parts. The support element is sealingly affixed to one end of a metal bellows whose other end is sealingly affixed to the body of the valve. The command-pressure chamber, i.e. the chamber to which the pilot pressure is supplied, is thus partly delimited by the bellows itself.
From a functional point of view, this valve represents a major advance over the valves previously described which rely on command cylinders. However, it contributes the additional drawback that in the normal functioning condition, namely, the condition under which the valve spends most of its time, i.e. the closed position, the bellows is distended. Thus to permit it to resist the pressure of the installation to be protected, the bellows must be made relatively thick, i.e. of significant wall thickness. This means that the bellows is also relatively stiff and fragile. In addition, the support element of the valve member must be of significant cross section and this contributes a correspondingly large dimensioning of the valve body.